This invention relates to a process for forming .epsilon.-caprolactone effectively (with high yield) while substantially inhibiting formation of by-products by supplying a percarboxylic acid solution obtained by oxidizing organic carboxylic acids in an organic solvent in the presence of hydrogen peroxide and a boric acid catalyst, and cyclohexanone in a reaction system so that the rates of hydrogen peroxide and the boric acid catalyst formulated are smaller, and effecting an oxidation reaction of cyclohexanone with percarboxylic acid in the reaction system.
Since a reaction mixture containing the .epsilon.-caprolactone obtained as described above contains substantially no by-product which is difficult to be separated and purified, .epsilon.-caprolactone having high purity can be obtained easily by a conventional purification process such as a distillation process.
In the prior art, it has been known that .epsilon.-caprolactone is prepared by reacting cyclohexanone with a percarboxylic acid such as peracetic acid and perpropionic acid (Baeyer-Villiger oxidation reaction). However, in the conventional preparation process, since various by-products such as adipic acid and 5-hexenoic acid are formed, it is extremely difficult to perform purification for isolating .epsilon.-caprolactone having high purity from a reaction mixture containing .epsilon.-caprolactone, and .epsilon.-caprolactone containing such by-products has exerted bad influence on production of polymers such as polyesterol and polyurethane.
As a purification process for isolating .epsilon.-caprolactone from a reaction mixture containing .epsilon.-caprolactone obtained by the above preparation process, there have been proposed various techniques, for example,
(1) a process in which low boiling point components of crude .epsilon.-caprolactone prepared by reacting cyclohexanone with a percarboxylic acid solution are distilled by a first distillation device, and then a product is distilled out by a second distillation device (Japanese Unexamined Patent Publication No. 34677/1981 and Japanese Unexamined Patent Publication No. 42684/1982).
(2) a process in which low boiling point components are removed by distillation from a reaction mixture obtained by oxidizing cyclohexanone, and then inert gas is introduced into a condenser of a second distillation column to prevent condensation of water (Japanese Patent Publication No. 59238/1985) and
(3) a process in which crude .epsilon.-caprolactone is treated with an acidic sulfite type anion exchange resin (Japanese Patent Publication No. 16437/1985). However, since these techniques require complicated purification steps or frequently form by-products (impurities) which cannot be separated by a purification operation such as a distillation operation in the above preparation process, there is a problem that they cannot be applied to a reaction mixture containing .epsilon.-caprolactone containing such impurities.
In recent years, there has been proposed a process in which in preparation of a percarboxylic acid to be used for oxidation reaction of cyclohexanone, a boric acid catalyst forming small amounts of by-products in preparation of .epsilon.-caprolactone is used in place of a strongly acidic catalyst such as sulfuric acid, and a percarboxylic acid solution obtained as a result is used as such for preparation of .epsilon.-caprolactone. Specifically, in Japanese Unexamined Patent Publication No. 150681/1982 and Japanese Unexamined Patent Publication No. 124781/1983, there has been proposed a process in which in preparation of stable .epsilon.-caprolactone by oxidizing cyclohexanone with percarboxylic acid having 2 to 4 carbon atoms, corresponding carboxylic acid and hydrogen peroxide are used in the form of a "crude solution of percarboxylic acid" obtained by reacting them in the presence of a boric acid catalyst and also while removing water continuously under azeotropic condition to prepare a stable .epsilon.-caprolactone solution. In this conventional process, since weak acid such as boric acid is used, smaller amounts of by-products are formed in preparation of .epsilon.-caprolactone when compared with the case where a strongly acidic catalyst is used, but a large amount of low boiling point components such as oxycaproic acid and high boiling point components such as an oligomer of 1,4-caprolactone, 5-hexenoic acid, ethyl propionicoxycaproate and ethyl oxycaproate are still formed as a by-product, whereby the process is not sufficiently satisfactory.
Accordingly, for preparing .epsilon.-caprolactone industrially by using percarboxylic acid and cyclohexanone, a preparation process which can inhibit substantially sufficiently amounts of by-products formed which are undesirable products in purification (impurities such as the low boiling point components and high boiling point components as described above) has been expected strongly.